Polyurethane elastic fiber nonwoven fabric and its production and synthetic leather using the same

ABSTRACT

A polyurethane elastomer fiber non-woven fabric contains polyurethane elastomer fiber filaments melt-bonded with one another, and has a tensile elongation of 100% or more, a 50% elongation recovery of 75% or more and a tear strength per METSUKE of 5.5 gf or more. The non-woven fabric can be prepared by a method of providing a thermoplastic polyurethane elastomer having Shore hardness A of 92 or more, drying the elastomer to a water content of 150 ppm or less, and melt-spinning and, at the same time, spraying it together with a high speed gas stream so as to deposit and laminate the elastomer into a sheet form. A synthetic leather using the non-woven fabric is also provided. The non-woven fabric has not only an excellent elasticity but also high tear strength.

TECHNICAL FIELD

The present invention relates to a polyurethane elastic fiber nonwovenfabric wherein polyurethane elastic fiber filaments are mutually fusedand bonded and the production thereof, and a synthetic leather using thepolyurethane elastic fiber nonwoven fabric and, more particularly, to apolyurethane elastic fiber nonwoven fabric having excellentstretchability and high tear strength and the production thereof, and asynthetic leather using the polyurethane elastic fiber nonwoven fabric.

BACKGROUND ART

Among polyurethane elastic fiber nonwoven fabrics made of polyurethaneelastic fiber filaments, a polyurethane elastic fiber nonwoven fabricproduced by a so-called “melt-blow spinning method” has hitherto beenused in the fields where comparatively good conformability to themovement of human body is required (for example, the side band of apaper diaper, a base fabric of an emergency adhesive plaster, adisposable glove, etc.) or the fields where comparatively shortstretchability is required (outerwear fields, for example, sportsclothes, stretchable cotton pads, etc.) because of excellentstretchability, flexibility and permeability.

Japanese Unexamined Patent Publication No. 6-293117 discloses that asynthetic leather having excellent stretchability is obtained by usingthis kind of a polyurethane elastic fiber nonwoven fabric as a basematerial for synthetic leather, and also suggests the use of such asynthetic leather as materials of shoes, bags, pouches, furniture,interiors, car sheets, and clothes.

However, since such a polyurethane elastic fiber nonwoven fabric hasexcellent stretchability and flexibility, as described above, butgenerally has a low tear strength, a sufficient tear strength as one ofthe important basic physical properties of the synthetic leather cannotbe attained when used as a base material of the synthetic leather.Depending on the purposes of the synthetic leather, products suitablefor practical use cannot be obtained.

Thus, an object of the present invention is to provide a polyurethaneelastic fiber nonwoven fabric having both excellent stretchability andhigh tear strength and the production thereof, and a synthetic leatherusing the polyurethane elastic fiber nonwoven fabric.

DISCLOSURE OF THE INVENTION

To solve the problems described above, the present invention provides apolyurethane elastic fiber nonwoven fabric comprising polyurethaneelastic fiber filaments which are mutually fused and bonded, wherein atensile elongation is not less than 100%, a recovery at 50% elongationis not less than 75%, and a tear strength per basis weight is not lessthan 5.5 gf, which is produced by dehydrating a thermoplasticpolyurethane elastomer having a Shore A hardness of not less than 92,thereby reducing the moisture content to 150 ppm or less, melt-spinningthe thermoplastic polyurethane elastomer, scattering the resultingpolyurethane filaments through a high-speed air flow, and depositing andlaminating the filaments in a sheet form at a temperature which is 35°C. lower than the Vicat softening temperature.

As described above, the polyurethane elastic fiber nonwoven fabric ofthe present invention is basically produced by a melt-blow spinningmethod. A general procedure of the melt-blow spinning method will now bedescribed simply below. That is, the melt-blow spinning method ischaracterized by feeding a molten thermoplastic polymer to nozzle holesarranged in a line, continuously extruding the molten polymer throughthe nozzle holes, blowing off a high-temperature gas from slits arrangedat both sides of the group of nozzles at high speed, thinning andsolidifying the polymer extruded through the nozzle holes by means ofgas energy to form continuous filaments, depositing and laminating thegroup of the continuous filaments on a moving conveyor net, and mutuallybonding the filaments due to self-bonding properties of the filamentsthemselves.

The first feature of the present invention is that the strength of thefilaments obtained by melt spinning is enhanced by using a hardthermoplastic polyurethane having a Shore A hardness of not less than 92as a thermoplastic polymer. The Shore A hardness means a standard of thehardness of a comparatively soft substance and is represented by a valuewithin a range from 0 to 100. On the other hand, the hardness of acomparatively hard urethane is represented by a Shore D hardness. Athermoplastic polyurethane, which can be produced industrially atpresent, is a product having a hardness within a range from a Shore Ahardness of about 70 to a Shore D hardness of about 75. Accordingly, the“thermoplastic polyurethane having a Shore A hardness of not less than92” in the present invention means a thermoplastic polyurethane, theupper hardness limit of which is up to a Shore D hardness of about 75.

A general thermoplastic urethane is produced by reacting a comparativelyhigh-molecular polyol component which forms a soft segment moiety, and alow-molecular diol and a diisocyanate compound which form a hard segmentmoiety. The thermoplastic polyurethane having a Shore A hardness of notless than 92 used in the present invention is a polyurethane containingthe hard segment moiety in a comparatively large amount and thosewherein the nitrogen content, which is a standard of the hardness, inthe polymer is about not less than 4% by weight corresponding to thethermoplastic polyurethane.

Although the soft segment component of the polyurethane includes, forexample, polyetherdiol, polyesterdiol and polycarbonatediol components,any component may be used in the present invention. Although thediisocyanate compound includes, for example, aromatic compounds,typically, 4,4′-diphenylmethane diisocyanate (MDI) and aliphaticdiisocyanates, typically, hexamethylene diisocyanate, any compound canbe used in the present invention.

If necessary, additives for the polymer, for example, hindered phenolantioxidants and various amine antioxidants; benzotriazole ultravioletabsorber and hindered amine ultraviolet absorber; smoothing agents suchas amide wax and montanoic acid wax; thermal decomposition inhibitorssuch as various carbodiimide compounds; various pigments such astitanium oxide and red iron oxide; and gas yellowing inhibitors may beadded.

Although the method of polymerizing a polymer includes, for example,various methods such as the continuous melt polymerization method, vatcuring method, belt method and kneader method, any polymerization methodmay be used in the present invention.

Although the hardness of the thermoplastic polyurethane basicallydepends on the mixing ratio of the respective components describedabove, the hardness varies with the ratio of the hard segment moiety tothe soft segment moiety, the length of the hard segment moiety, and thestate of phase separation. The thermoplastic polyurethane used in thepresent invention is preferably a polyurethane wherein the hard segmentmoiety is long and is contained in a large amount and also the Vicatsoftening temperature is high. Specifically, as described above, athermoplastic polyurethane having a Shore A hardness of not less than 92and Vicat softening temperature of not lower than 120° C. is preferablyused.

The second feature of the present invention is that the thermoplasticpolyurethane is dehydrated to reduce the moisture content to 150 ppm orless, preferably 110 ppm or less, more preferably 70 ppm or less, andmolten before melt-blow spinning. The reason is as follows.

The “hard thermoplastic polyurethane having the Shore A hardness of notless than 92” used in the present invention has such characteristicsthat the melting point is higher than that of the thermoplasticpolyurethane used in a conventional melt-blow nonwoven fabric and themelt density is drastically high. For example, the melt viscositymeasured at 190° C. of the thermoplastic polyurethane having the Shore Ahardness of 85 is within a range from about 5,000 to 50,000 poise,whereas, the melt viscosity of the thermoplastic polyurethane used inthe present invention is within a range from 100,000 to 1,000,000.

By the way, since the molten polymer is drawn to a proper fiber diameterby heating air in the melt-blow spinning process, the melt viscosity onextrusion through a nozzle must be drastically reduced regardless of thecomposition of the polyurethane. Accordingly, although the thermoplasticpolyurethane having a high viscosity used in the present invention mustbe spun at very high temperature during melt-blow spinning, thethermoplastic polyurethane is thermally decomposed, vigorously, when itis molten at high temperature. It is well known that a urethane bond isthermally decomposed, vigorously, when the temperature exceeds 230° C.When the melt-blow spinning of the thermoplastic polyurethane is carriedout under conditions of vigorous thermal decomposition, the moltenpolymer extruded through the nozzle foams, thus making it impossible toform a filament, in the worst case.

Under the conditions of comparatively less thermal decomposition, anonwoven fabric can be obtained without causing yarn breakage, but theresulting nonwoven fabric has poor mechanical properties because ofreduction of the tensile strength. The cause for deterioration ofmechanical properties is deemed as follows. That is, the thermoplasticpolyurethane chemically changes during melting, thereby to reduce themolecular weight or to cause insufficient crystallization andrearrangement of the hard segment moiety after formation of thefilament.

Thus, according to the present invention, the above-described adverseinfluence exerted as a result of the use of the high-hardness polymercan be reduced to the minimum by reducing the moisture content of thethermoplastic polyurethane to 150 ppm or less, preferably 110 ppm orless, more preferably 70 ppm or less, before melt-blow spinning.

The third feature of the present invention is that filaments aredeposited and laminated in a sheet form at a temperature which is 35° C.lower than the Vicat softening temperature. The reason is as follows.

The polyurethane elastic fiber nonwoven fabric, which is obtained bymelt-blow spinning of the thermoplastic polyurethane used in the presentinvention, presented such a phenomenon that mutual bonding of thefilaments exerts an influence not only on the tensile strength, but alsoon the tear strength. Enhancement of a mutual bonding force between thefilaments usually increases the tensile strength, while decreasing thetear strength, though a polyurethane elastic fiber nonwoven fabric doesnot always behave in the manner described above. Therefore, it becomesnecessary to optimize the mutual bonding force between the filaments toobtain a nonwoven fabric having a good balance between the tensilestrength and the tear strength.

The most effective method of controlling the mutual bonding forcebetween the filaments is to control the depositing point temperature ofthe filaments in case the molten polymer extruded through the nozzle isdeposited and laminated on the conveyor net while thinning and coolingby a high-temperature gas flow. This depositing point temperature varieswith the melting temperature, the temperature of the high-temperaturegas, or the flow rate, and the condition capable of exerting a largestinfluence on the depositing temperature is a distance between the nozzleand the conveyor net. If other conditions are the same, the depositingpoint temperature of the filaments decreases when the distance betweenthe nozzle and the conveyor net increases. Suction of the heating airunder the conveyor net is also an important condition to control thedepositing point temperature of the filaments. The larger the suctionamount of the heating air under the conveyor net, the more thedepositing point temperature of the filaments decreases. The suctionamount is preferably set to an amount which is two times, morepreferably three times, larger than the flow rate of the heating air.

The depositing point temperature can be set by using the Vicat softeningtemperature of the urethane polymer as the standard. When the filamentsare deposited in the state where the temperature is higher than theVicat softening temperature, a film-like nonwoven fabric is obtainedbecause of insufficient solidification of the filaments. When thefilaments are cooled to a temperature lower than the Vicat softeningtemperature, the mutual bonding force between the filaments is reducedvirtually in proportion to the difference between the temperature andthe Vicat softening temperature.

Accordingly, according to the present invention, high tear strength ofnot less than 5.5 gf is attained in place of reducing the tensilestrength to some degree by depositing and laminating the filaments aftercooling to the temperature so that a difference in temperature becomeshigher than 35° C. It becomes possible to further enhance the tearstrength by subjecting to various treatments such as lamination,softening, heat treatment, embossing and the like.

As described above, the nonwoven fabric having a high tear strength ofthe present invention can be obtained by the method of subjecting ahigh-hardness polyurethane to melt-blow spinning in the state where achemical change is reduced to the minimum, thereby to comparativelyreduce the bonding force of filaments on deposition and to maintain alarge degree of freedom of the filaments, which constitute the nonwovenfabric. The reason why the tear strength is enhanced by maintaining alarge degree of freedom of the filaments is deemed as follows. That is,since a lot of filaments participate in the portion to be broken,one-point concentration of stress does not occur.

The fiber diameter of the filaments, which constitute the polyurethaneelastic nonwoven fabric of the present invention, is preferably within arange from about 5 to 50 μm. The smaller the fiber diameter, the moreflexible the resulting nonwoven fabric becomes. When the fiber diameteris smaller than 5 μm, the tear strength is reduced.

The polyurethane elastic nonwoven fabric of the present invention canrealize a synthetic leather having both excellent stretchability andhigh tear strength, wherein a tensile elongation is not less than 100%,a recovery at 50% elongation is not less than 75%, and a tear strengthper basis weight is not less than 2.5 gf, preferably not less than 3.0gf, by laminating a skin layer made of a urethane elastomer onto thesurface of the polyurethane elastic nonwoven fabric as a base material.The basis weight of the nonwoven fabric is usually within a range fromabout 25 to 500 g/m², but is preferably from about 50 to 400 g/m² whenused as the base material of the synthetic leather.

A conventional synthetic leather has a structure wherein a skin layermade of a polyurethane is laminated onto a woven knitted fabric ornonwoven fabric made of non-stretchable fibers, and such a syntheticleather is mainly produced by two kinds of methods. The first method isa wet method of regenerating a urethane layer in water from a urethanesolution coated on a fiber layer as a base material, while the secondmethod is a dry method, typically a method of coating a skin layerproduced in a separate step with an adhesive and laminating the coatedskin layer onto a fiber layer as a base material. Among these two kindsof methods, the dry method is suited for the method of producing thesynthetic leather of the present invention. The synthetic leatherproduced by the dry method generally has such a structure that apolyurethane surface layer having a thickness within a range from about10 to 70 μm is firmly bonded onto a fiber base material having athickness within a range from about 0.3 to 1.5 mm via an adhesive layerhaving a thickness within a range from about 20 to 150 μm.

The skin layer used in the synthetic leather of the present inventioncan be produced by the method of coating a crested release paper with aurethane resin solution for skin and drying the urethane resin solutionin a dry oven or the method of drying and solidifying a slurry solutionor dispersion of a polyurethane resin for skin to form a sponge-likeskin.

To bond the polyurethane elastic fiber nonwoven fabric as the basematerial with the skin layer, adhesive solutions composed mainly of apolyurethane resin having reactivity, a crosslinking agent, and areaction accelerator, which are generally used, can be used. Among theseadhesive solutions, an adhesive solution using only a polar solventcapable of remarkably dissolving a urethane such asN,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc) or the like isnot preferred because it easily dissolves the urethane elastic fibernonwoven fabric of the present invention.

In the present invention, an aqueous adhesive which does not dissolve aurethane, an adhesive solution using a solvent (e.g. methyl ethyl ketone(MEK), toluene, etc.) which does not dissolve a urethane but swells theurethane, or an adhesive solution using a mixed solvent of thesesolvents and DMF or DMAc is preferred. A urethane hot-melt adhesivecapable of bonding without using any solvent is particularly preferredin the present invention. Although the hot-melt adhesive is roughlyclassified into a spray type adhesive which is coated to the adhesiveportion in a molten state and is capable of bonding until the adhesiveis solidified, and the other type adhesive which is molten and bondedwith heating after interposing a solid adhesive in a state of a nonwovenfabric, any type may be employed.

With respect to the synthetic leather of the present invention, the skinlayer may be bonded after physical properties of the nonwoven fabricwere improved by subjecting the polyurethane elastic fiber nonwovenfabric before laminating the skin layer onto a treatment such as heatpress, needle punching, rubbing or impregnation with an elastic polymersolution.

The synthetic leather of the present invention can be produced, forexample, by the following manner. First, a crested release paper iscoated with a urethane resin solution for skin in a coating weight ofabout 50 to 200 g/m² using a coater and the urethane resin solution isdried in a hot oven. Then, the skin layer is coated with a urethaneadhesive solution in a coating weight of about 100 to 300 g/m² and,after sufficiently drying the solvent, a polyurethane elastic fibernonwoven fabric fed by a pressure roller is laminated and nipped,thereby properly impregnating the polyurethane elastic fiber nonwovenfabric with the adhesive. The reactive adhesive is reacted by aging inthe state, and then the release paper is removed and the resultingsynthetic leather is taken up. In case the degree of the reaction of theadhesive is insufficient, the reaction is completed by aging at 60 to100° C. It is preferred to select a material having good stretchabilityand excellent permeability as the skin layer and adhesive used in thepresent invention.

The synthetic leather of the present invention has a three-layerstructure composed of a urethane stretchable nonwoven fabric, anadhesive layer and a skin layer, and physical properties of a fiberlayer as the base material greatly contribute to physical propertiessuch as tensile elongation, elongation recovery and tear strength of thesynthetic leather having such a structure. As a similar structuresynthetic leather having stretchability, for example, a syntheticleather obtained by laminating a skin layer onto a nylon tricot havingstretchability or a stretchable woven fabric as the base material isknown. These synthetic leathers have large tensile strength and hightear strength, as a feature of a nylon filament product, but havedrawbacks that the tensile elongation is small and the elongationrecovery is drastically lowered. On the other hand, the syntheticleather of the present invention has excellent stretchability and largetear strength of not less than 2.5 gf per basis weight.

The synthetic leather requires the tear strength of not less than 2.5 gfper basis weight and the reason lies in durability when applying thesynthetic leather to uses, for example, shoes, sheets and furniture. Forexample, when pinholes are formed, breakage easily occurs from pinholesin case of a synthetic leather having a low tear strength, while thesynethetic leather having a tear strength of not less than 2.5 gf,preferably not less than 3.0 gf, per basis weight does not present sucha phenomenon that breakage results from pinholes.

The synthetic leather of the present invention has such a surprisingperformance wherein clear embossing, that could have never beenperformed in the case of a conventional synthetic leather, can becarried out and a clear emboss thus formed does not disappear, even ifsevere elongation recovery is repeated and, furthermore, a portionhaving a very small curvature can also be embossed and the embossed formis not impaired by a strong outer force. Such a surprising performanceis deemed as a reflection of a proper conformability capable ofconforming to any form of the polyurethane elastic fiber nonwoven fabricof the present invention and excellent thermosetting properties of thehard segment moiety of a comparatively hard polyurethane.

INDUSTRIAL APPLICABILITY

The synthetic leather of the present invention is fit for use as, forexample, sports shoes used in tennis, golf, and track and field eventsby utilizing excellent stretchability and flexibility described above,and can also be fit for uses as, for example, furniture and sheetmaterials by making use of embossability and integral moldability. Itcan be said that the synthetic leather is also fit for uses as, forexample, clothes such as outerwear by making use of excellent drapingquality, in addition to the flexibility and stretchability.

What is claimed is:
 1. A non-woven polyurethane fabric comprisingpolyurethane elastic fiber filaments which are mutually fused and bondedand having a tensile elongation of not less than 100%, a recovery at 50%elongation of not less than 75% and a tear strength per basis weight ofnot less than 5.5 gf, said fabric being produced by dehydrating athermoplastic polyurethane elastomer having a Shore A hardness of notless than 92 to reduce the moisture content thereof to no more than 150ppm, melt-spinning the thermoplastic polyurethane elastomer anddepositing and laminating the filaments in a sheet form at a temperaturewhich is 35° C. lower than the Vicat softening temperature.
 2. Asynthetic leather comprising a base material layer made of a non-wovenpolyurethane fabric comprising polyurethane elastic fiber filamentswhich are mutually fused and bonded and having a tensile elongation ofnot less than 100%, a recovery at 50% elongation of not less than 75%and a tear strength per basis weight of not less than 5.5 gf, saidfabric being produced by dehydrating a thermoplastic polyurethaneelastomer having a Shore A hardness of not less than 92 to reduce themoisture content thereof to no more than 150 ppm, melt-spinning thethermoplastic polyurethane elastomer and depositing and laminating thefilaments in a sheet form at a temperature which is 35° C. lower thanthe Vicat softening temperature.